Electrical machines comprising a high-voltage winding are used to a large extent in various applications in networks for transmission and distribution of electricity. Examples of non-rotating machines of this kind are transformers and reactors.
High voltage in this context means voltages in excess of 1 kV.
In addition to comprising a high-voltage winding, known transformers also comprise a low-voltage winding, and conventionally the high-voltage winding and the low-voltage winding are arranged around a core of magnetic material. Further, insulating layers are arranged at least between the core and one of the windings and also between the windings. The insulating layers often consist of paper impregnated with oil. One disadvantage of these prior art insulating layers is that they have to be made thick to function satisfactorily. Another disadvantage of handling such layers is that it entails a risk of contamination. These problems can be avoided by using solid insulating layers. One example of a transformer with solid insulation is described in the international application with publication No. WO 97/45847. This transformer has a cable wound around a core of a magnetic material. The transformer solves the problem of leakage of oil that is hazardous to the environment. The same technique may be used for manufacturing other non-rotating electrical machines, such as, for example, reactors.
In many cases there is a relative shortage of space at the locations where a transformer is to be placed. This is true, for example, in those cases where the transformer is to be placed in a densely populated area or inside a building. In such cases, it would be desirable to have a less bulky transformer or a transformer with a geometrical shape that is adapted to the space available. The transformer may then, for example, be located in an existing cable channel, along a wall, or below a roof. In many cases, it is also desirable to provide a transformer with a lower weight, for example when the transformer is to be placed on top of a power-line pylon.
When distributing current to private dwellings, it is desirable to step down the voltage to ordinary mains voltage as late as possible to minimize the losses. Usually, the voltage is then stepped down from a voltage of the order of magnitude of 10 kV to 400 volts. In many countries, it is customary to place such transformers at the top of a pylon. However, because of the size of the transformers, there is a risk that they may blow down, which results in costs as well as maintenance and repair work. Also in this case, it is desirable to minimize the size of the transformer.
In many cases, it is desired to connect a cable to the high-voltage winding on a transformer according to the above. Such a cable conventionally comprises a conductor surrounded by an insulation. The connection of the high-voltage winding to the electric cable may be performed in many different ways. However, it is important to avoid high electric fields during the connection, since these could lead to electrical breakdown.
Thus, there is a need of an electrical machine with smaller dimensions or with a geometrical shape different from that of currently used machines, so that the above-mentioned problems can be avoided while at the same time avoiding high electric fields when connecting a cable to the high-voltage winding.
One example of an electrical machine that solves many of the above-mentioned problems is described in applicant's Swedish application 0003037-9 (not published), which is incorporated herein by reference.
In applicant's above-mentioned application 0003037-9, the connection of the cable is made by inserting the cable between the insulating layers of the transformer, whereby the cable conductor is connected to the high-voltage winding. A problem that arises when making such a connection is the high electric field that may arise in the region where the high-voltage winding is terminated and where the insulation of the transformer changes into the insulation of the cable, that is, in the cable termination. This high electric field may result in electrical breakdown to the outside of the transformer. To control the electric field in the cable termination region, the first and second insulating layers of the transformer have therefore been provided with so-called corona protection layers in the region for the cable connection. These layers have a non-linear resistivity as a function of the electric field, and their function is to equalize the electric field. In certain applications, for example in applications with high-voltage distributions with steep voltage derivatives at high frequencies, it would, however, be desirable to have an alternative to the corona protection layers. The reason for this is that heat is built up in the layers while at the same time the voltage distribution varies for different frequencies.
Thus, there is a need of an electrical machine with a design that differs from that of currently used machines, so that the problems mentioned above can be avoided also in high-voltage applications with steep voltage derivatives at high frequencies.